The “Amazing” feature of the giant panda has been developed for at least six million years.

Ailurarctos Reconstruction
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Ailurarctos Reconstruction

Reconstruction of Ailurarctos from Shuitangba. Its pseudo-thumb grip (shown on the individual right) has reached the level of modern pandas, while the radial sesamoid may have gone slightly more than its modern analogue during walking (the individual appears on the left). Credit: Illustrated by Mauricio Anton

Eating bamboo? It’s all on the wrist.

When is the thumb not the thumb? When a giant panda used to catch bamboo has an elongated wrist bone. Throughout its long evolutionary history, the panda’s hand has never really developed its thumb. Instead, he developed a thumb-like figure from a bony wrist, a radial sesamoid. This unique adaptation helps these bears (members of the Carnivora gang or meat eaters) live entirely on bamboo, even though they are bears.

In a new article published today (June 30, 2022), scientists say they have discovered the earliest bamboo-eating ancestral panda with this “thumb”. Surprisingly, it is longer than modern generations. The study was conducted by Xiamong Wang, a curator of vertebrate paleontology at the Museum of Natural History in Los Angeles, and colleagues.

Popular fake thumb in modern giant pandas (Ailuropoda melanoleuca) Has been known for more than 100 years, and it has not been understood how this wrist bone evolved due to the almost complete absence of fossil records. Fake thumb fossil from ancestral giant panda, AilurarctosIt was found 6-7 million years ago in the Shuitangba area in Zhaotong, Yunnan Province, southern China. This gives scientists a first look at the early use of this additional (sixth) figure – and the earliest evidence of a bamboo diet in ancestral pandas – to help us better understand the evolution of this unique structure.

Chengdu Panda eats bamboo

Chengdu panda eats bamboo. Credit: Reproduction of the photo with the permission of Sharon Fisher

Curator of NHM Vertebrate Paleontology Dr. Xiaoming Wang. “Keeping bamboo trunks to biting size is probably the most important adaptation for consuming large amounts of bamboo.”

How to walk and chew bamboo at the same time

This discovery could also help solve a panda’s enduring mystery: why aren’t their false thumbs so developed? As the ancestor of modern pandas, Ailurarctos it could have been expected to have a less developed false “thumbs up,” but Wang and his colleagues discovered a fossil that found a false thumb that was shorter, longer than the hooked figure of the modern generation, and had a more straight tip. So why did the pandas’ false thumbs stop growing to get a longer figure?

“A panda’s false thumb should walk and chew,” says Wang. “Such a binary function serves as a limit to how big this ‘thumb’ can be.”

Panda Gripping vs Walking

Panda catch and walk (white bone is a lying thumb). Credit: With permission from the LA County Museum of Natural History

Wang and his colleagues believe that the short lying thumbs of modern pandas are an evolutionary compromise between the need to manipulate bamboo and the need to walk. The hooked tip of the modern thief’s second thumb allows them to manipulate the bamboo, while also transferring their impressive weight to the next bamboo dish. After all, the “thumb” performs a double function like a radial sesamoid – a bone in the animal’s wrist.

“Five to six million years should be enough time for a panda to develop longer lying thumbs, but apparently the evolutionary pressure of the need to travel and carry weight kept the ‘thumb’ short – strong enough to be useful without being big enough. Denise Su, an associate professor at the School of Evolution and Social Change and a research scientist at the Institute of Human Origins at Arizona State University and co-leader of a project to restore panda specimens, says.

“Pandas, who evolved from a carnivorous ancestor and became purebred bamboo eaters, have to overcome many obstacles,” says Wang. “The ‘thumb’ facing the wrist may be the most astonishing development against these obstacles.”

Reference: “The earliest giant panda offers conflicting requirements for thumb movement and feeding” by Xiaoming Wang, Denise F. Su, Nina G. Jablonski, Xueping Ji, Jay Kelley, Lawrence J. Flynn and Tao Deng, June 30, 2022 year, Scientific Reports.
DOI: 10.1038 / s41598-022-13402-y

The authors of this article are affiliated with the Museum of Natural History, Los Angeles County, Los Angeles, CA, USA; Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; Arizona State University, Tempe, Arizona, USA; Pennsylvania State University, University Park, Pennsylvania, USA; Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China; Yunnan Institute of Cultural Deposits and Archeology, Kunming, Yunnan, China; Harvard University, Cambridge, Massachusetts, USA.

Funding was provided by the US National Science Foundation, the Yunnan Natural Science Foundation, the Chinese National Natural Science Foundation, the governments of Zhaotong and Zhaoyang, and the Institute of Vertebrate Paleontology and Paleoanthropology.

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